Wood processing equipment such as chippers and flakers generally rely on fast moving cutting blades or knives being moved past stationary wood pieces to produce chip or flake products. As best shown in FIG. 1, which is a schematic view of an exemplary cutting disc arrangement, knives 10 are mounted to a rotating disc 12 that rotates in the direction of arrow 11 past an anvil surface 14. Wood pieces 16 are supported on anvil 14, and the cutting disc is advanced in the direction of arrow 13 into the wood pieces so that knives 10 cut flakes or chips from the wood pieces. For safety reasons, it is desirable to have a sensing system 18 in place to detect if the rotating knives come in contact with the anvil either directly or indirectly by means of foreign material in the wood pieces. The sensing system acts as an early warning system so that if parts do make contact as they move past each other, the system can quickly and reliably detect such an event and shut down the machinery promptly to mitigate further damage to the equipment. In the case of the cutting disc arrangement shown in FIG. 1, an event such as loosening of a cutting knife 10 resulting in movement out of the rotational plane of the disc and contact with the anvil would trip the sensing system. In an alternative scenario, foreign material such as metal in the wood piece may be picked up by the knife and brought into contact with the anvil to activate the sensing system. In both situations, the sensing system acts to stop advancement of the cutting disc, partially retract the disc, and initiate a shutdown to prevent further damage. Given the high inertial loads of the rotating disc, the shutdown of the equipment can take several minutes.
Previous methods and systems for detecting contact between parts include proximity sensors, however, these tend to require careful calibration and are often unreliable due to lack of resolution.
Existing contact sensor systems known to the applicant also include the use of light based detectors that detect a laser or LED light beam through a light tube positioned in a region where parts move relative to each other. For example, in the typical arrangement illustrated in FIG. 1, the light tube would be positioned on the leading edge 18 of the anvil 14 adjacent the rotating knives 10. If the knives or foreign material should contact the anvil and in turn deform the light tube, light will be prevented from reaching the detector thereby signaling a collision requiring quick shutdown of the wood processing equipment. Due to the operating environment in which the sensor is positioned, light based systems tends not to operate reliably. Moisture ingress due to high vibration cycles or condensation due to temperature fluctuations tends contaminate the interior of the light tube to block the light and produce a false collision signal. The light tube arrangement also requires a reliable light source as any intermittent failure of the light will also be detected as a false collision. Light based system are also susceptible to reliability issues due to a need for tuning of the light detector as it is an analog device that requires a potentiometer to be ‘dialed’ during setup. If calibrated incorrectly, the sensor may miss collisions or detect non-existent collisions.
False or missed collision detections in any system are undesirable. False collision detections result in unnecessary shutdown of the equipment which can be time consuming to restart with resulting expensive losses in production. Missed collision detections result in damage to the equipment that may be expensive to repair or replace.